SELECT 
  feature_id, 
  variant_id, 
  value, 
  value_int 
FROM 
  cscart_product_features_values 
WHERE 
  product_id = 504 
  AND lang_code = 'en'

Query time 0.00036

Explain
id select_type table type possible_keys key key_len ref rows Extra
1 SIMPLE cscart_product_features_values ref lang_code,product_id product_id 3 const 22 Using where

Result

feature_id variant_id value value_int
30 123
127 5895
128 5899
129 5906
149 0 5905400520026
178 6315